The Elite Hagen thermostat is a good one for that size. My tank is 20 and I have the same one. Just make sure you keep the thermometer within view and adjust the temperature according to the thermometer's guide. I tend to keep the temperature lower based on the thermostat guide rather than higher.
If the fish are showing signs of sickness or stress, I've heard that is when you kick the temperature up more and treat water with a Stress Enyzmes or Milafix for sick and ick.
Are you going to "school" in some other fish or stick with Mollies? Dalmation Mollies are pretty cool.
The tank looks great Bina.